Oneness, Uniqueness of Allah (Tawheed)

Bukhari :: Book 9 :: Volume 93 :: Hadith 485

Narrated 'Abdullah bin 'Amr:

Abu Bakr As-Siddiq said to the Prophet "O Allah's Apostle! Teach me an invocation with which I may invoke Allah in my prayers." The Prophet said, "Say: O Allah! I have wronged my soul very much (oppressed myself), and none forgives the sins but You; so please bestow Your Forgiveness upon me. No doubt, You are the Oft-Forgiving, Most Merciful."